#include <iostream>
#include <vector>

using namespace std;

int v[1000][1000];

int main()
{
    int n, m;
    cin >> n >> m;
    int ret = 0;
    for (int i = 0; i < n; ++i)
    {
        for (int j = 0; j < m; ++ j)
        {
            if (v[i][j] == 0)
            {
                ++ret;
                if (i + 2 < n)
                {
                    v[i + 2][j] = -1;
                }
                if (j + 2 < m)
                {
                    v[i][j + 2] = -1;
                }
            }
        }
    }
    cout << ret;
    return 0;
}